The present invention relates to a battery pack for a cordless power tool, and more particularly, to a battery pack that stores rechargeable or secondary lithium battery cells. The present invention also relates to a cordless power tool having the battery pack.
In a cordless power tool such as a driver, a drill, and an impact tool, rotation of a motor is decelerated by a deceleration mechanism, and the decelerated rotation is transmitted to an end tool. Conventionally, a commercial power source (alternating current source) is used as a power source to the motor. However recently, rechargeable alkaline batteries such as nickel-cadmium battery cells and nickel hydride battery cells are used as a power source of a cordless power tool.
In accordance with an increase in a required voltage in the cordless power tool, the number of battery cells to be accommodated in a battery pack is increased. For example, since a nominal voltage of nickel-cadmium battery cell is 1.2V, 12 battery cells must be accommodated in the battery pack if the required voltage is 14.4V, and 20 battery cells must be accommodated in the battery pack if the required voltage is 24V, and the battery pack must be attached to a cordless power tool. Therefore, in accordance with an increase in required voltage, entire weight of the power tool is increased.
On the other hand, an organic electrolyte rechargeable battery such as a lithium battery cell and lithium ion battery cell provides a higher nominal voltage. Therefore, the number of the battery cells can be reduced. As a result, a compact cordless power tool can be provided.
Here, the lithium battery cell is a generic term over a vanadium-lithium battery and a manganese-lithium battery, and has a negative electrode made from a lithium-aluminum alloy employing organic electrolyte. Further, the lithium ion battery cell has a positive electrode formed of cobaltate lithium and a negative electrode formed of graphite employing organic electrolyte. Throughout the specification, organic electrolyte rechargeable battery such as the lithium battery cell and the lithium ion battery cell will be simply referred to as “lithium battery cell”.
The lithium battery cell provides high nominal voltage, for example, 3.6V equivalent to three nickel-cadmium battery cells. That is, the number of battery cells can be reduced by employing lithium battery cells. This is advantageous in terms of a demand of an increase in capacity, and reduction in size and weight. However, degradation of lithium battery cell occurs if excessive charging and/or excessive discharging is performed, or if excessive electric current flows through the lithium battery cell. As a result a service life of the lithium battery cell will be reduced. Further, gas may be generated due to the decomposition of the electrolyte if excessive charging is performed. Moreover, degradation of characteristic occurs due to excessive discharging, which in turn causes electrical short-circuit in the battery cell upon subsequent charging.
To avoid this problem, Japanese Patent Application Publication No. 2003-164066 discloses a protection circuit and control process for protecting the battery cells. The circuit includes a switching element such as a field effect transistor (FET) connected between the battery pack and a DC motor. For the protection, the switching element is shut off prior to the excessive discharging. Relevant technique is also described in Japanese Patent Application Kokai Nos. H11-55866, H04-75430, 2002-223525, and 2000-12107.